The present invention relates generally to medical imaging systems and, more particularly, to a radio frequency (RF) receiver coil array for a magnetic resonance (MR) imaging system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is oscillating at a radio frequency that is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of nuclear magnetic resonance (NMR) signals are received by a RF coil array and subsequently digitized and processed to reconstruct the image using one of many well known reconstruction techniques. With respect to the RF coil array, MR systems often include a dedicated receiver coil array that is integrated into a patient table or formed as a separate surface coil, with the receiver coil array comprising a two-dimensional array formed from a plurality of coils.
One imaging procedure for which a dedicated receiver coil array is typically employed is breast imaging. The majority of dedicated receiver coil arrays currently used for breast imaging employ an “open” coil design in order to accommodate for interventional procedures during an imaging procedure. That is, most commercially available breast coils are constructed so as to provide a physician at least one of lateral and medial access to the breasts of an imaging patient, such that a breast biopsy can be performed during the imaging procedure. Due to the open design, these breast coils sacrifice signal-to-noise ratio (SNR) and parallel imaging capability in order to allow for accessibility to the breast. Such commercially available breast coils are comprised of larger coil elements, which leads to a bigger noise volume, reducing the SNR.
Several commercially available “closed” coil designs also exist for breast imaging, where the coils are constructed to substantially surround the breast, thereby not providing access to the breasts during imaging. However, such coil designs use a one-size-fits-all policy where the coils are over-sized so as to fit larger size breasts. Such closed coils are inefficient for imaging small and medium breasts, as the breasts of a patient may be separated from the coils by a distance large enough to affect image quality, such as by reducing the SNR.
For both the existing open and closed breast coil designs, inefficiencies exist due to the design of the coil. That is, limitations of the coil geometry in such coils regarding how closely the coils can be placed relative to the breasts of the patient lead to reduced SNR and capability of parallel image acceleration in only one dimension. This leads to reduced image quality and increased scan time.
It would therefore be desirable to have a receiver coil array for breast imaging where the coils are placed closer to the breast, so as to increase SNR. It would also be desirable that such a coil array provide for parallel image acceleration in two dimensions, so as to reduce scan time and provide as much spatial independence of the coils as possible, so as to enable the highest possible parallel imaging acceleration factors to be used.